1. Field of the Invention
The present invention relates to a white organic light emitting device, and more particularly, to a white organic light emitting device emitting light having a color which is not affected by a change in current, has excellent luminescence efficiency, and long lifespan.
2. Description of the Related Art
Organic light emitting devices are self-emissive devices which use electroluminescence, and have high recognition ability, and are complete solid devices having excellent impact resistance. Therefore, organic light emitting devices have gained a great deal of attention and have been used in a variety of different kinds of display devices.
Organic light emitting devices basically have an anode/organic emissive layer/cathode structure, and can further comprise a hole injection layer, a hole transport layer, an electron injection layer, or the like. For example, the structure of anode/hole injection layer/hole transport layer/organic emissive layer/electron transport layer/cathode, and the structure of anode/hole injection layer/hole transport layer/organic emissive layer/electron transport layer/electron injection layer/cathode are known.
Recent development effort has focused on organic light emitting display devices, and in particular, white organic light emitting devices.
White organic light emitting devices are organic light emitting devices that emit white light, and can be used for various different applications such as paper-thin light sources, backlights for liquid crystal display devices, or light sources for full-color display devices employing color filters.
A method of forming an emissive layer of a white organic light emitting device can be categorized into two types. One is a method of forming a single emissive layer, and the other is a method of forming emissive multi-layers.
A single emissive layer can be prepared using a single material or by doping or blending at least two types of materials. For example, the single emissive layer can be formed using red and green dopants with a blue emissive host, or using red, green and blue dopants with an emissive host material having a large band gap energy. However, energy transfer into dopants generally is inefficient, and therefore is incomplete. Also, the single emissive layer can be formed using a bipolar host material having red, green, or blue luminescence moiety. However, adjusting the color balance that results in white emission can not be easily adjusted.
A white organic light emitting device comprising the emissive multi-layers can be a 3-wavelength white organic light emitting device comprising a red emission layer, a green emission layer and blue emission layer; or a 2-wavelength white organic light emitting device using complementary colors for red, green and blue.
For the 2-wavelength white organic light emitting device that uses complementary colors for red, green and blue, high efficiency can be obtained. However, white is obtained using the complementary colors, and thus realizing full colors using color filters is difficult and the range of colors that can be expressed is narrow. Whereas, in the case of the 3-wavelength white organic light emitting device, due to energy transfer between molecules, a uniform spectrum of three colors, that is, red, green and blue, can not generally be achieved and therefore light emission efficiency remains low.
Korean Patent Publication No. 2005-0028564 discloses a method of manufacturing a white organic light emitting device, comprising: doping any one pigment selected from green and red on a portion of or the whole of a surface of any one of a hole transport layer and an electron transport layer that are formed on upper and lower surfaces of a blue emission layer; and doping the other one of the green and red pigment on the other layer of the hole transport layer and the electron transport layer. In addition, Japanese Patent Laid-Open Publication No. 2005-150084 discloses a white organic light emitting device in which a double hole blocking layer comprising in order a first hole blocking layer disposed on a surface of the anode, a hole transport layer disposed on a surface of the first hole blocking layer opposite the anode, and a second hole blocking layer is formed on a surface of the hole transport layer opposite the first hole blocking layer, and an emissive layer is formed on a surface of the second hole blocking layer opposite the hole transport layer; the structure provided thereby has high color purity and luminance in spite of the structure of the emissive layers comprising a green emission layer, a blue emission layer and a red emission layer formed in this order. The white organic light emitting device is manufactured using a simple manufacturing process, but it still generally has a low light emission efficiency and color purity.
Meanwhile, when each emissive layer comprises only a phosphorescent material, excellent efficiency can be obtained even though the white organic light emitting device has a short operating lifespan due to instability of a blue phosphorescent material which further creates limited color stability. In particular, in the case of a white organic light emitting device which uses a blue phosphorescent dopant that is excited via a conductive host, the exchange energy loss is large in terms of power efficiency (i.e., because the energy loss during exchange is large, power efficiency is low), and a change in luminescence spectrum due to a change in current is also severe.
Korean Patent Publication No. 2005-0074208 discloses an organic electroluminescence device comprising a first electrode, a second electrode and an emissive layer structure that is disposed between the first electrode and the second electrode, the emissive layer structure comprising both a fluorescent emissive layer and a phosphorescent emissive layer disposed on the fluorescent, wherein the fluorescent emissive layer is an emissive layer that emits light of a blue wavelength. However, in such an organic electroluminescence device, the change in luminescence wavelength positions as a function of current change is large, and the internal luminescence efficiency in each of the fluorescent and phosphorescent emissive layers is high. Therefore, change in the luminescence spectrum as a function of current change is still high.
Yiru Sun et al. in Nature, 2006, vol. 440, p. 908 discloses an organic light emitting device manufactured such that a blue emissive layer that uses a fluorescent material is formed on both outer surfaces of an emissive layer structure, a spacer layer comprising only a host material is disposed on an inner surface of each of the blue emissive layers, and green and red emissive layers are disposed sequentially on the inner surfaces of the spacer layer such that the green and red emissive layers are disposed between both of the spacer layers, and as such the organic light emitting device has improved luminescence efficiency and increased lifespan.
FIG. 1 is a schematic cross-sectional view illustrating the structure of an emissive layer (“EML”) 10 disposed between opposing surfaces of a hole transport layer/electron blocking layer (“HTL/EBL”) 15 and an electron transport layer/hole blocking layer (“ETL/HBL”) 16 of a conventional white organic light emitting device. Referring to FIG. 1, a blue emissive layer 13 is formed at both outermost surfaces of an emissive layer structure, a spacer layer 14 is formed on an inner surface of each of the blue emissive layers 13, and a red emissive layer 11 and a green emissive layer 12 are formed and disposed between the two spacer layers 14.
However, when the Schottky barrier of the electrodes is not equal, current flowing from the electrodes varies as a function of the change in voltage applied. Accordingly, in an emissive layer structure having a nonsymmetrical structure, luminescence spectrum varies according to current distribution.
Since current density as applied from both electrodes is not the same at all times, exciton transfer into blue, green and red emissive layers from an anode, and exciton transfer into the blue, green and red emissive layers from a cathode is not symmetrical. As a result, large changes in color can occur as a function of changes in current.